1. Technical Field
This invention relates to a modulation system suitable for radio links, especially satellite links. Such links have limited power and bandwidth that puts a ceiling on the data throughput. Increasingly, bandwidth is the scarcer resource so that often there is a power margin. There is continuing effort in the modem industry to provide higher throughput over satellite links in order to lower the transmission costs. Developments include the use of 8PSK trellis coded modems.
2. Related Art
Satellite communications have been a very important part of the global telecommunications backbone for the past three decades. Satellite operators provide Internet Services Providers (ISPs) with long distance point-to-point (trunk) links to the Internet backbone. Currently, approximately 4% of satellite transponders are used for Internet trunking services. Although optical fibre connections are increasingly being used in the mature communications markets, satellite remains the only option (or at least the only cost-effective option) in other less developed or remote areas. Satellite communications have a number of specialist applications, notably news-gathering, military, emergency services.
Satellite news-gathering is becoming increasingly important with the popularity of TV news, particularly ‘rolling news’ channels such as CNN, Sky News and BBC News 24. The demand for the rapid transmission of up-to-date reports from international trouble spots has grown sharply. However, the quality of the broadcast images and sound are often poor because of the limited bandwidth caused by lack of capacity. Indeed, capacity is particularly likely to be limited in the very regions where newsworthy events such as major world conflicts or natural catastrophes occur, both because of disruption to the infrastructure and because of the needs of the military or emergency services. The ability of the proposed modulation scheme to increase capacity by a factor of two to three times would significantly enhance the quality of news broadcasts from remote regions.
There has been a huge increase in demand for bandwidth in the military sector, due to new strategies of ‘network-centric warfare’. The fundamental principle of these strategies is to be able to conduct military operations with a numerically inferior force, by ensuring that all members of that force have a full and up-to-date picture of both the enemy and their own forces. The result of these strategies is that intelligence assets (mainly aircraft and special forces) are generating very large quantities of sensor data (especially imagery) that need to be rapidly transmitted to command and control systems and then to weapons systems. Typically, satellite communications met 70% of the total bandwidth requirements, with terrestrial wireless and optical fibre networks providing the rest. There is thus a very significant driver from the military for more efficient use of satellite transponder capacity.
Global demand for satellite Internet trunking is predicted to approximately double from 2005 to 2010. An improved modulation scheme would enable more efficient use of satellite transponders for this growing communications demand. Given the high capital cost of satellite systems, there is significant incentive for service providers to use transponders very efficiently.
The present invention relates to improvements to frequency modulation (FM) schemes. International patent specification WO03/069867 describes a multi-level Gaussian frequency-shift keying (MGFSK) modulation scheme in which the impulse response extends over adjacent symbols. A block diagram of this system is shown in FIG. 1.
The modulator section 11-14 begins by mapping the incoming bits 10 to an alphabet of sixteen symbols (block 11), which equates to four bits per symbol. The sixteen symbols are represented by different amplitudes symmetrical about zero, e.g., −7.5, −6.5, −5.5 . . . −0.5, +0.5 . . . +5.5, +6.6, +7.5. These are multiplied by impulses of value 1 (block 12) to produce impulses of −7.5, −6.5 . . . +6.5, +7.5 at point ‘x’.
These are applied to a Gaussian filter 13 to produce a set of impulses having a Gaussian shape, i.e. smooth waveforms with no negative or oscillatory characteristics, which are applied to a frequency modulator 14. This method of modulation is very similar to the scheme used by the GSM air interface, except that GSM uses only 2 levels (+/−1). The bandwidth of the Gaussian filter 13 is a critical parameter, if it is set smaller the occupied bandwidth is reduced but the intersymbol interference is increased since it sets the width (in time) of the impulse response. With this scheme, the product of bandwidth b and symbol period t, has been set at 0.35 as a reasonable trade-off. In comparison, the GSM mobile telephony standard uses a product bt=0.3.
The modulation index of the FM modulator is held at β=1, which according to Carson's rule means that the occupied bandwidth can be considered as 2*fm, where fm is the maximum frequency component of the modulating (baseband) signal which is set by the bandwidth of the Gaussian filter. Under this condition the occupied bandwidth BW can be calculated to beBW=Symbol rate*bt*2=Symbol rate*0.7 (Hz)and the (uncoded) bandwidth efficiency E is E=4 (bits/symbol)/0.7=5.71 (bits/sec)/Hz. Since this is a narrow-band FM scheme it has a constant envelope, and is therefore suited to low-cost transmitters where the output amplifier can be saturated and also to satellite channels where the satellite transponder can be saturated. In this system the demodulator is based on look-up tables that train themselves according to the channel characteristics, which are determined by sending a unit impulse into the channel and measuring the response at the receivers. This system offers a significantly improved bandwidth efficiency compared to earlier 8PSK (8-level phase shift keying) systems, and is comparable to 64QAM. The use of 64QAM over satellite is not practical because to support it the channel would have to be highly linear and well equalised.
Typical parameters for MGFSK for a satellite application are the use of sixteen levels, giving 4 bits per symbol (because 16=24), and partial response signaling, in which the symbols are formed by the impulse response of a low-pass Gaussian filter that stretches over adjacent symbols, similar to that in use with GSM. This enables a reduction in occupied bandwidth at the expense of some inter-symbol interference (ISI) and loss of orthogonality. The combination of multiple levels and partial response signaling results in a bandwidth efficiency of almost 6 (bits per sec)/Hz
The filtered waveform is frequency modulated (block 14) onto a carrier 15 using a low modulation index (β) in order to keep the occupied bandwidth in the narrow-band class. Because it uses frequency modulation (FM) instead of the more common PSK, the signal is tolerant to equalization errors and non-linearities in RF amplifiers. These parameters are designed to maximize the bandwidth efficiency and, at the same time, maintain a constant envelope and the use of FM to maximize tolerance to non-linearities and equalization error.
A simulated 16-GFSK spectrum is shown in FIG. 2. In FIG. 2, the carrier frequency held low for the purposes of simulation at 2 kHz and the symbol rate is 10 baud. The 3 dB bandwidth (shown by the dotted horizontal line) of this spectrum is just 7 Hz. When scaled to 400 Mbit/s, the symbol rate is 100 Mbaud and the bandwidth would be 70 MHz.
As shown in FIG. 1, the demodulator 16-19 consists of an FM demodulator 16, filtering 17, a symbol detector 18, and a bit recovery processor 19. The filter 17 is designed to band-limit the noise but at the same time preserve a well-behaved impulse response, to ensure that it does not add significantly to the inter-symbol interference. The actual filtering implementation is performed in two stages at different sampling rates. The waveform at ‘y’ in FIG. 1 should be as close as possible to the waveform at the output of the Gaussian filter 13 in the modulator.
A simple detector 18 has been devised which makes use of look-up templates that store expected waveforms for every possible symbol transition (256 templates) and its operation is illustrated in FIG. 3. The input from the demodulator 16, 17 is read in (31) and compared (step 32) with templates which have been loaded previously (step 35). The symbol agreeing most accurately with the templates is selected (step 33) and generated as an output 36. The selection of the templates to be loaded (step 36) is made according to the output 36 for the previous symbol. Thus the number of templates to be compared is the same as the number of possible symbols “n”, but the templates are selected from a larger corpus n2 corresponding to all possible transitions between from symbol to the next.
In this prior art system it is relatively straightforward to allow for inter-symbol interference in the first half of the symbol because that half is more affected by previous symbols, which have already been determined and can be corrected for. However, it would be desirable to be able to use any part of the symbol, as there is no particular reason why the latter half should be any less prone to inter-symbol interference than the first half.
The entire set of templates can be built from the unit impulse response of the channel, since linearity can be assumed to hold with this scheme even though the channel is highly non-linear, because FM is used over the channel. The templates can be re-built at regular intervals if needed from the impulse response or from training sequences.
This scheme provides an increased bandwidth of two to three times that currently available commercially with quadrature amplitude modulation (QAM) schemes. The system provides best advantage when used in high bandwidth links that occupy the whole transponder. This provides a potential throughput of over 400 Mbits/s through a conventional 72 MHz transponder. The modulation system is particularly suited for point-to-point high-bandwidth services to large receiving antennas of 5 meters or more, for use in internet backbone connections, and in satellite news-gathering. It may also be suitable for smaller dish services such as television broadcast and provide a viable and better alternative to the traditional QAM schemes used, for example, in digital video broadcasting. The performance of a simulation of such a detector, which has a memory of one symbol, is plotted on FIG. 5, together with the theoretical plots for QPSK and 64-QAM for easy comparison. The error patterns are very different from those experienced with QAM schemes, in that when the symbol detector makes mistakes due to noise, the detected symbols are wrong only by +/−1 symbol and the errored symbols frequently appear in complementary pairs.